Venezuelan prosecutors say they would charge opposition figure Juan Guaido with “high treason” for plotting to hand over a disputed oil-rich border area to foreign companies, in exchange for political support from Britain.

Attorney General Tarek William Saab said on Friday that Guaido is being investigated for negotiating to renounce “the historical claim our country has on the territory of Esequibo” — which is controlled by neighboring Guyana.

Saab asserted that Guaido was engaged in “illegal negotiations, behind the back of the country” in exchange for “political support from the United Kingdom.”

“The facts imply a crime of treason,” he added.

The case is based on audio recordings purported to involve a US administration official urging an adviser to Guaido to “deliver the Esequibo” to the oil company Exxon Mobil and other multinationals, according to Vice President Delcy Rodriguez who released the file.

The 159,000 square kilometer territory is the subject of a long-standing border dispute between Venezuela and Guyana, which is a member of the British Commonwealth.

The US has, in the meantime, deployed a military contingent to Guyana for the first time in a decade.

In the audio file, UK envoy Vanessa Neumann is heard speaking with the UK Foreign and Commonwealth Office, saying that the opposition should “drop the topic” of Venezuela’s claim to the territory in order to secure London’s political support.

According to Rodriguez, the recording was made before Guaido suddenly declared himself “interim president,” in January.

The opposition figure plunged the country into political turmoil by rejecting the outcome of the May 2018 election, which President Nicolas Maduro won. He has been accusing Maduro of “usurping power” and calling on him to step down.

He was immediately recognized by the United States and its Western allies as the legitimate leader of the oil-rich nation.

He had parliamentary immunity as a member of the National Assembly, even though the opposition-held body was earlier dissolved. Guaido already faces several other charges, including one of “usurping the functions of the president.”

Washington has warned Venezuela authorities against any attempt to arrest Guaido.